Hard surfacing of steels

ABSTRACT

It has been found that when certain steels, such as the hot work die steels which contain relatively high amounts of silicon, are surface hardened by the diffusion of boron into their surfaces, an undesirable soft layer is formed immediately subjacent thereto. When the silicon content is reduced to a value less than about 0.5 percent, the soft layer is reduced in thickness and at about 0.05 percent it no longer is formed.

United States Patent Inventor Howard C. Fiedler Schenectady, N.Y.

Appl. No. 889,730

Filed Dec. 31, 1969 Patented Dec. 21, 1971 Assignee General Electric Company HARD SURFACING 0F STEELS 3 Claims, No Drawings US. Cl 148/315 Int. Cl C23b 5/00,

C22c 39/14 Field of Search 148/3 I .5, 6,

[56] References Cited UNITED STATES PATENTS 3,024,176 3/1962 Cook 204/39 3,151,001 9/1964 Garofalo 148/31.5 X 3,222,228 12/1965 Stanley et al. 148/315 X Primary Examiner-Charles N. Lovell Att0rneys-Richard R. Brainard, Paul A. Frank, Charles T.

Watts, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman HARD SURFACING F STEELS CROSS-REFERENCE Cross-reference is made to U.S. Letters Pat. No. 3,024,176 in the name of Newell C. Cook, assigned to the present assignee, entitled "Corrosion Resistant Coating" issued Mar. 6, 1962.

This invention relates to the formation of hard, wear and corrosion resistant surfaces on steel bodies and more particularly to such surfaces formed by the difl'usion of boron into the surfaces of such bodies.

Hard and corrosion resistant surfaces have been formed on the surfaces of iron and steel bodies by the diffusion of boron into such surfaces. However, it has been observed that in certain instances, a soft layer is produced under the hard outer surface which is undesirable because it has caused the outer layer to crack and spall off when the surface is subjected to certain kinds of loading. It would be desirable to prevent the formation of this soft layer and thereby prevent this kind of failure of the hard layer and such is a principal object of this invention.

Briefly stated, and in accordance with one embodiment of the invention, it has been found that when iron-base alloys, such as steel, for example, containing more than about 0.5 percent by weight silicon are subjected to a surface-hardening treatment by the diffusion of boron into the surfaces, iron boride is formed in the hardened surface layer and an objectionable soft layer is formed subjacent to the hardened layer. This soft layer promotes the fracture and spalling off of the hardened layer when it is subjected to loading such as, for example, by attempting to perform a penetration hardness measurement thereon. Obviously, when this occurs, the softer underlayer is exposed and the benefits of improved wear and corrosion resistance is lost. When the silicon content of these alloys is reduced to values below about 0.5 percent, the thickness of the soft underlayer is reduced to acceptable levels and when it reaches about 0.05 percent by weight, the formation of the soft layer is eliminated.

While the formation of hard, wear and corrosion resistant surface on an iron base alloy body by the diffusion of boron therein to form hard iron borides may be accomplished in several ways, a preferred method is that disclosed by the previously referenced patent, the disclosure of which is incorporated by reference herein. Briefly, this technique, broadly known as metalliding," is accomplished by electrolyzing a fused bath of a boron compound using the iron alloy object to be surface hardened as the cathode and graphite or another relatively inert electrical conductor as the anode.

More specifically, and as a particular working example, a metalliding bath was employed composed of a molten mixture of about mol percent potassium fluoride, mol percent sodium fluoride and 50 mol percent lithium fluoride as an electrolyte containing about 0.2 mol percent boron trifluoride. A number of specimens of hot work die steels consisting essentially of, by weight, about 0.35 to 0.45 percent carbon, about 0.25 to 0.35 percent manganese, less than about 0.5 percent silicon, about 4.0 to 6.0 percent chromium, about 0.35 to 1.05 percent vanadium, up to about 1.5 percent tungsten, about 1.25 to 1.60 percent molybdenum and the balance iron were treated therein, the steels having the following nominal compositions:

TABLE I Nominal composition (weight percent) 0 Mn Si Cr V W Mo The molten salt bath with the dissolved boron trifluoride was contained in a Monel vessel and an atmosphere of 90 percent nitrogen and 10 percent hydrogen was employed. Boron was deposited on the surface of the metal specimens by the electrolytic discharge of boron ions from the solution. The deposition rate, which is controlled by the current density, was maintained below the rate at which the boron is absorbed by the steel so that no elemental boron formed on the surface. The cell was operated between about and percent coulombic efficiency.

pered for l hour at 540' C., all under argon. When an attempt was made to measure the hardness of the resulting iron boride hardened surfaces of the specimens on the Rockwell C" scale, the indentor caused the surface surrounding and under the indentor to severely crack and craze. When these specimens were sectioned and examined metallographically, a soft continuous layer was found to exist between the hard outer layer and the core. For example, a sample of H13 commercial tool steel which was borided at 890 C. for 183 minutes during which time the current density had decreased from 1.55 amperes per square decimeter to 0.25 amperes per square decimeter followed by heat treatment, had a hard outer bonded layer of about 30 microns in thickness and a subjacent soft layer of about 18 microns in thickness. The hardness of the outer layer, which consisted of Fe,B, was about 2,000 Knoop, of the soft layer about 450 KHN and the core about 670 KHN, all under a IOO-gram load. Examination of the specimen under an electron microprobe indicated virtually no silicon in the boride layer, up to almost 4 percent silicon in the soft layer, and a little over 1 percent silicon in the core.

Samples of H13 of the nominal composition, i.e., containing about 0.9 percent silicon, H13 with only 0.5 percent and 0.05 percent silicon were borided for from 3 to 3.5 hours at 900 C. and heat treated as before. The current density was 1.0 ampere per square decimeter for the first half of each run and 0.5 ampere per square decimeter for the balance of each run. When subjected to a Rockwell C" penetration hardness test, hardness numbers of 54.5, 52.5 and 51 were obtained for the 0.9, 0.5 and 0.05 percent silicon alloys, respectively. Severe spalling occurred around the indentations in the 0.9 percent silicon alloy, some spalling in the 0.5 percent silicon alloy and none in the 0.05 percent silicon alloy. Under metallographic examination, all three alloys had an outer hard layer about 35 to 40 microns in thickness, the 0.9 percent silicon alloy had a continuous subjacent soft layer of about one-third to one-half the thickness of the hard layer, the 0.5 percent silicon alloy had a continuous soft layer of about one-half the thickness of the soft layer of the 0.9 percent alloy, and the 0.05 percent silicon alloy did not have a soft layer at all.

From the foregoing, it would appear that such steels, i.e., the chromium hot work die steels, after boriding and heat treatment have a soft layer of ferrite beneath the boride layer which is about one-third the thickness of the latter layer. Because of the high silicon content, this soft layer does not transfonn to austenite during the heat treatment and thus cannot be hardened. With silicon contents below about 0.5 percent, the soft layer is reduced in thickness or eliminated entirely.

What 1 claim as new and desire to secure by Letters Patent of the United States is:

1. An article of manufacture comprising a body of chromium-hot-work-die-steel having at least one surface portion, said surface portion comprising a hard surface layer consisting essentially of iron boride, and said steel consisting essentially of,

said silicon content is less than about 0.1 percent.

3. An article of manufacture as set forth in claim 1 wherein said body is characterized by the absence of a substantially continuous layer of ferrite subjacent to said iron boride layer. 

2. An article of manufacture as set forth in claim 1 wherein said silicon content is less than about 0.1 percent.
 3. An article of manufacture as set forth in claim 1 wherein said body is characterized by the absence of a substantially continuous layer of ferrite subjacent to said iron boride layer. 